Over long distance (WAN) on a tuned system, WinSCP 4.3.1 using SFTP (protocol ssh-2) caps around 690kbytes** on download and upload. Cipher AES or blowfish does not make a significant difference. (Ciphers make a large difference during local performance where CPU is more of an issue.)

We found that switching to Filezilla removed the download cap, reaching up to 4.4MB/s (40mbps - which was near the limit of the line). Upload was capped at 650kbytes, which has since been removed by the Filezilla devs (Fixed as of 3.3.5)

I can only make an educated guess and say that I think the problem lies with a too small socket receive buffer.

More research:
I found another tool that has adjustable send/receive buffers. The default it negotiated was 1024kbytes, which was still slow. The ideal for this particular line seems to be 2048kbytes. My guess is that WinSCP has 512 send/receive buffers. Those should at minimum be upped to 1024 in general, and consider makeing that a user adjustable parameter since its useful particularly on large and long lines, aka WAN.

Re: Slow downloads using SFTP

Update: I had a bit of software that had a default of 128kbytes, which autotuned to 1024kbytes for socket receive buffer. I then set default to 2048, then it autotuned to 4096. Autotuned = tcp sliding window adjustment, in the socket receive buffer. That's for download only, that particular bit of software performed poorly when it came to upload, so no good numbers.

I hate to put you on the spot but do you consider this a priority item or a low level item?

Re: winscp 4.3.2

Re: winscp 4.3.2

Hi - came across this thread while googling this issue. Just to add my .02 -- I have 3.8.2 installed side by side with 4.3.2 on the same machine - 3.8.2 gets 4300 KB/s while 4.3.2 gets 30 B/s while trying to sftp upload to the same server.

Re: winscp 4.3.2

Hi - came across this thread while googling this issue. Just to add my .02 -- I have 3.8.2 installed side by side with 4.3.2 on the same machine - 3.8.2 gets 4300 KB/s while 4.3.2 gets 30 B/s while trying to sftp upload to the same server.